This invention relates to a rotary anode type X-ray tube and more particularly to an improvement of the X-ray tube wherein the difference of temperature between the inner and outer rings of the roller bearing which supports the rotary shaft for rotation of the target is reduced, thereby assuring prolongation of lifetime and reduction of noise.
The target of the rotary anode type X-ray tube is heated to a very high temperature due to the impingement of high speed electron beams. Typically, the target is connected to a cylindrical rotor and rotated thereby. The rotary shaft for rotation of this target also served as a central rotary shaft of the rotor and is rotatably mounted, by way of roller bearings, on a stationary cylindrical housing concentrically inserted in the cylindrical rotor.
The diameter of a connector shaft between the target and the rotor is made as small as possible so as to minimize heat transfer. However, a large amount of heat is transferred to the rotor, and the rotary shaft and rotor are still heated to a high temperature. On the other hand, the amount of heat transferred from the rotary shaft to the housing is small because of the interposition of the roller bearing, thereby maintaining the housing at an extremely low temperature relative to the rotary shaft and rotor in a conventional X-ray tube of rotary anode type. Since the housing is at a lower temperature relative to the rotary shaft and rotor, the temperature of the inner ring of the roller bearing which is in direct contact with the rotary shaft is higher than that of the outer ring of the roller bearing which is in contact with the housing by about 250.degree. C. for example. In consideration of the difference of thermal expansion due to the difference of temperature during operation, the roller bearing needs to have a clearance of about 60 microns when the X-ray tube is not operated or at the beginning of the operation at which the inner and outer rings are at the same temperature. It is known that a large clearance in the bearing for supporting the rotary shaft rotating at so a high speed as in the rotary anode type X-ray tube causes not only vibration and noise, but considerable reduction of service life of the bearing due to the material fatigue.